The decision to plug and abandon a well or field is often based on simple economics. Once production value drops below operating expenses, it is time to consider abandonment, even if considerable reserves remain. It is also useful to plug and abandon a well to use an existing slot to sidetrack into new payzones. This process is known as “slot recovery” and is very cost effective compared to drilling a new complete well. Consequently, plug and abandonment (P&A) is an inevitable stage in a lifespan of a well.
In a typical P&A operation, operators remove existing completion hardware, set plugs and squeeze cement into an annulus at specified depths across producing and water-bearing zones to act as permanent barriers to pressure from above and below. Operators remove the wellhead last. One of the main problems in any cementing procedure is contamination. Poor mud-removal in areas where the cement is to be set can give rise to channels through the plug caused by the drilling fluid. To avoid this, a spacer is often pumped before and after the cement slurry to wash the hole and to segregate the drilling fluid and the cement from each other.
Different regulatory bodies have their own requirements for plugging operations. Most require that plugs be placed and tested across any open hydrocarbon-bearing formations, across all casing shoes, and across freshwater aquifers. More and more, metal or alloy plugs are used in place of or in conjunction with cement to meet these requirements.